shiva1

I'll get my car back tomorrow. Had timing belt service, all pulleys and tensioner checked, new boots for c/v joints, fresh brake fluid and tranmission fluid topped off. I installed a new (rebuilt) LH unit last month. Car should be ready to venture into the high rev zone now. Prior to service I got a timing belt tension warn as I was puling into the driveway after about a 20 mile easy cruise. After much consideration and letting the car idle for extended periods time while I looked at the belt though the vent tube of the belt housing I decided to drive the car slowly to my mechanics shop. I made it okay but on the way I had a wierd thing happen. As I was gently pulling away from a traffic light the motor just quit! I had enought momentum to pull over to the curb and I was sure that I had just ruined 32 valves and I now had a parts car. After a minute of wringing my hands and trying not to cry, I summond the courage to turn the key. The motor fired instantly and idled perfectly. I let it idle awhile and decided to drive it gently keeping an eye out for the belt tension light. Uning gentle throttle I made it to the shop with no warning light, no problems. The motor did not miss a beat. I'll talk to the mechanic tomorrow to see if he has a possible explanation.

Randy S.
'87 928S4 Automatic
Diamontblau/Burgandy

danglerb

Sounds electrical, wire or sensor. Does your mechanic have a diagnostic tool, spanner or whatever that talks to the brains?

jcorenman

Randy,

For its day the 928 systems were quite advanced. As Arthur C. Clarke pointed out, "Any technology that is sufficiently advanced is indistinguishable from magic". I think the car was just having a bit of fun with you.

And I also think you did the right thing, gave the belt the careful eyeball through the vents and then drove it to the shop.

Now get it back from the mechanic and drive it like you stole it! (And get the belt tension checked in 1500 miles or so).

Mike Frye

You said you put in a new (rebuilt) LH a month ago. Maybe the harness or something else down there wasn't seated properly?

Landseer

Randy, sounds from reading your posts you have a good mechanic to support the car.
Did you have him release the flexplate tension, take a baseline measurement of crankshaft endplay and install a supplementary clamp, being as its an automatic 32V car?

S4ordie

OT a little bit. During that work did you have all the rubber fuel lines replaced? This is critical to the safety and longevity of the car and you.

shiva1

Got the car back today after timing belt service, tensioner, belt pulleys checked out okay. New c/v joint boots, oil and brake fluid changed. A brief tour at 100 mph to help blow some carbon out on the way home. Car pulls strong up to 5500 rpm, got nervous about going higher. After about an hour of highway driving,pulled up to a light. Car died twice while idling in drive. Once underway, no problems at speed. Mechanic's hunch is an air leak somewhere. Not tended to I feel this is indicative of worse to come. Suggestions?

'87 S4
Automatic
95 k miles

Landseer

You didn't answer the first time, maybe you thought my question was superfulous?

Has the mechanic been routinely releasing flexplate tension and measuring crankshaft endplay during your stewardship? Please say yes.

shiva1

Hi Lanseer,

Sorry I didn't answer your question, it wasn't superflous-it's just me and my ignorance. I don't know the concept of flexplate tension and how it relates to? and therefore did not ask my mechanic. Please explain to one less mechanically informed.
Update on my car idle problem. Yes I was correct. The problem persists and occurs with increased frequency. Car dies after a short while at idle now. Damn, time to park it and back to trouble shooting. Perhaps new LH unit I installed is the source? Loose connections? Module lock mechanism loose? Engine vacuum/air leak? Thanks to all for your input.

Randy S.
'87 S4
Diamantblau

Landseer

They are complex cars. Vac leaks and electrical glitches can take them down.

I had dirty ground connections (there are a dozen or so) on a 32V 85 that caused the car to miss and die sometimes. The two major ground cables (bat to frame; engine to frame) can deteriorate. The ground for computers to engine has also caused car to intermittently die. The advice is polish each ground and inspect cables annually. My battery to frame connection looked good, but the flat cable had corroded beneath the plastic shroud and would cause the car to stall.
The other grounds that were worst offenders were buried at the back of the valley of the motor under the fuel lines. Its the electrical ground for computers and other associated componentry.

So those are a few things that you must do, esp. if not done during your 8 or so years ownership. Leaking intake boots / vac leaks / grounds throughout car / ground cables.
Hopefully its something like that.


Unfortunately, the S-4 automatics are also plagued with another more ominous design flaw that manifests itself in the kind of warm-stall symptom you've described. And its not an uncommon situation. Some mechanics don't know about it. Basically, the back of the engine has flexplate mounted on the crankshaft, onto which a clamp is mounted, which in turn grips the driveshaft. Pressure builds up, caving the flexplate, bowing it forward, putting pressure inside the engine.

Normal cars use a sliding connection. Porsche pins the connection rigidly, but not rigidly enough.

The situation is described in this forum extensively, so search for "thrust bearing failure" (using the quote marks in the search box).

So, people here with S4 automatics release the tension by loosening and retightening the single pinch bolt that holds the driveshaft splines in the clamp. When loose, they pry the crankshaft fore/aft slightly to measure movement, called endplay. And, they usually install an auxilliary or specially designed replacment clamp grips tighter to prevent the driveshaft from sliding. (The problem is that stock clamp has proven itself not strong enough. The shaft splines creep out of the clamp on heavy accel, then don't slide back into the clamp, causing a pressure build up that manifests into wear within the engine. In a sense, the drive shaft grows in length ).

A lot of owners don't know about this potential issue. The release of tension is preemptive. Auxilliary or special replacement clamps are applied with the tension released.

First real runability symptom is warm stalls.

Y'all have a high concentration of these cars in the northwest. Your mechanics see them and understand them. If a mechanic knows the 928 enough to be able to work on it, then he could know about this issue and should hopefully have engaged you in dialogue about it long, long ago.

No matter what the root cause of your stalling problem, you should make sure this tenion release / end play baseline measurement is made. Keep fingers crossed its not the problem.

John Speake

It would be wise to upgrade your MY 87 LH and EZk ECUs with MY88 EPROMs to allow a diagnostic tool to be used. The problem may be a stuck idle control valve (ISV). It would be easy to check this out with a diagnostic tester.

Does the idle speed hunt up and down at all ? Is the idle speed 675rpm when engine is at normal temperature ?

shiva1

Landseer and John. It's great to have access to the wealth of knowledge about the car from people like you. Due to the high cost of parts and labor I am learning to do more of the work myself although I have so far only installed a LH brain. I think next project wiil be a basic tune up with help from all 928 people.
Landseer-I talked to my mechanic about flex plate issues and grounds. He is really leaning toward air leak issues. Next up back to the shop -smoke test for air leaks.
John-motor idles steady @ 675-700 rpm. Probably not idle control valve.
I'll keep you posted. Thanks again.
P.S. John-cost to upgrade to MY88 EPROM?

Imo000

I think you mechanic is leaning towards the " I have no idea what's wrong with it" issue. Use the forum and start testing everything yourself or else your mechanic will be going to Hawaii on your $$$$.

shiva1

That is always a possibility but 35 years of Porsche experience and 8 years of service are hard to completely rule out.

Landseer

The first thing your mechanic needs to do on your next visit is release the flexplate pressure, whether he thinks so or not, and provide a reading on endplay. This is not a "one car in 10,000" type of oddball situation. Its a design flaw that emerged early, was glossed over a bit, and is now beginning to really affect some cars.